1. Field of the Invention
The present invention concerns in general a process for polymerizing/curing diepisulfides which comprises cationic photopolymerization of diepisulfides and the use of such a process for making optical lens materials, in particular optical lens materials having refractive indices of 1.7 or more.
2. Description of Prior Art
Document U.S. Pat. No. 5,807,975 discloses thermal polymerization and curing of diepisulfides in particular for the manufacture of optical materials such as a lens material for spectacles. This thermal polymerization/curing of diepisulfides lasts for several hours, generally about 20 hours.
Photo-initiated cationic polymerization of monoepisulfides is disclosed in the article xe2x80x9cPhotocationic crosslinking of poly(2,3-epithiopropyl methacrylate) and Photoinitiated cationic polymerization of its model compoundsxe2x80x9d M. Tsunooka et al, Journal of Polymer Science: polymer chemistry edition, vol. 22, 2217-2225 (1984). However, the photoinitiators used in the photopolymerization are colored material that are not suitable for making optical lens materials.
Cationic polymerization of diepisulfides to form polymer networks has not been reported. The diepisulfides show different reactivities than monoepisulfides due to the neighboring episulfide functional groups.
It, thus, exists a need for fast processes for polymerizing dicpisulfides and diepisulfide base compositions for the manufacture of lens materials having ultra high refractive indices of 1.7 or more with good optical properties.
It is an object of the present invention to provide a process for cationic photopolymerization of diepisulfides.
It is another object of the invention to provide a process for making in a relatively short time an optical lens material by cationic photopolymerization of a diepisulfide base composition.
It is still another object of the invention to provide a process for making an optical lens having a refractive index equal to or greater than 1.7.
These and other objects are provided by a process for polymerizing/curing a polymerizable monomer composition including at least one diepisulfide monomer which comprises mixing to the composition an effective amount of at least one cationic photopolymerization initiator (photoinitiator) to obtain a photopolymerizable composition and irradiating the photopolymerizable composition with an UV radiation to at least partially cationically photopolymerize the photopolymerizable composition.
In a preferred embodiment, the process for polymerizing/curing the photopolymerizable composition comprises first preheating the photopolymerizable composition to a predetermined temperature, irradiating with a UV radiation to cationically partially polymerize the photopolymerizable composition and then heating the partially polymerized composition to a predetermined temperature for a predetermined time to complete polymerization and cure.
It was found that the above process is particularly useful for making plastic lenses of high optical quality, both finished or semi-finished, in a relatively short time.
In particular the above process can be used for making high optical quality lenses which are clear, hard, processable and free of striations.
Thus, the present invention also provides a process for making plastic lenses in particular having a refractive index of 1.7 or more, which comprises the steps of:
a) pouring in a mold a photopolymerizable monomer composition including at least one diepisulfide monomer and at least one cationic photopolymerization initiator;
b) preheating the photopolymerizable monomer composition to a temperature ranging from 50 to 100xc2x0 C.;
c) irradiating the photopolymerizable monomer composition with a UV radiation to partially polymerize the composition;
d) heating the partially polymerized composition at a temperature ranging from 30 to 100xc2x0 C. for a predetermined time to complete polymerization; and
e) recovering the plastic lens from the mold.
During the UV irradiation step, the polymerization temperature is preferably monitored so that this temperature is kept within a predetermined range. For controlling the exothermic reaction during photopolymerization, the exposure to UV radiation is repeatedly turned on and off as needed to maintain constant the polymerization temperature which in turn results in a uniform polymerization or curing.
According to the invention, the photopolymerizable monomer composition includes at least one diepisulfide monomer.
Suitable diepisulfide monomers are diepisulfides of formula: 
in which R1 and R2 are, independently from each other, H, alkyl, aryl, alkoxy, aryloxy, alkylthio or arylthio; R3 and R4 are, independently from each other, 
Ra designates H, alkyl, aryl, alkoxy, aryloxy, alkylthio or arylthio and, n is an integer from 0 to 4 and m is an integer from 0 to 6.
A preferred class of diepisulfides is comprised of diepisulfides of formula: 
in which R1, R2, R3 and R4 are defined as above.
Another preferred class of diepisulfides is comprised of diepisulfides of formula: 
in which R1, R2, R3 and R4 are defined as above.
In R1, R2, R3 and R4 the alkyl and alkoxy groups are preferably C1-C6, more preferably C1-C4 alkyl and alkoxy groups such as methyl, ethyl, propyl, butyl, methoxy, ethoxy, propoxy and butoxy.
preferred diepisulfides are those of formula: 
The photopolymerizable monomer compositions also include at least one cationic photoinitiator for cationic photopolymerization of the monomers of the composition.
Any cationic photoinitiator can be used in the photopolymerizable composition of the invention, but preferably the photoinitiator shall not color the resulting photopolymerized material.
Among the preferred photoinitiators, there may be cited the compounds of formula: 
where X is Sb or P, 
and mixtures thereof.
Cationic photoiniators are commercialy available under the tradenames of RHODORSIL(copyright) 2074 (compound of formula V), Irgacure(copyright) 261 (compound of formula IV), CYRACURE(copyright) (mixed triarylsulfoniumhexafluoroantimonate salts and mixed triarylsulfoniumhexafluorophosphate salts of formulac I and III), SATCAT(copyright) (mixed triarylsulfoniun and iodonium salts) and TTAS (compound of formula VI).
In general, the photoinitiator is used in an amount of 0.005 to 5% by weight based on the total weight of the polymerizable monomers of the composition and preferably in an amount of 0.25 to 1% by weight.
The photopolymerizable compositions of the invention may include a photosensitizer. Any known photosensitizer may be used in the composition of the invention.
Among usual photosensitizers, there may be cited alkoxyacetophenones, benzoinethers, phosphine-oxides, benzoyloximes, benzophenones, benzyls, xanthones, anthrones, thioxantones, fluorenones, suberones, acridones and anthracene.
Such kind of photosensitizers are known in the art and described in particular in US patents U.S. Pat. No. 4,026,705; U.S. Pat. No. 4,069,054 and U.S. Pat. No. 4,250,053.
Preferred photosensitizers are selected from the group consisting of:
a) aromatic tertiary amines having the formula: 
where Ar1, Ar2 and Ar3 are aromatic groups having 6 to 20 carbon atoms; Z is selected from oxygen, sulfur, 
xe2x80x83a carbon to carbon bond; or 
where Rxe2x80x21 and Rxe2x80x22 are selected from hydrogen, alkyl radicals of 1 to 4 carbon atoms and alkenyl radicals of 2 to 4 carbon atoms and wherein n1 is zero or 1;
b) aromatic teriary diamines having the formula: 
where Y is a divalent radical selected from arylene and Ar8xe2x80x94Zxe2x80x94Ar9, where Z is as described above; Ar4, Ar5, Ar6, Ar7, Ar8 and Ar9 are aromatic groups having 6 to 20 carbon atoms; and wherein n1 and m1 are zero o r 1; and;
c) aromatic polycyclic compounds having at least three fused benzene rings and further having an ionization energy less than about 7.5 cV.
d) fluorescent polyaryl compounds selected from the group consisting of polyarylenes, polyarylpolyens 2,5-diphenylisobenzofurans, 2,5-diarylfurans, 2,5-diarylthiofurans, 2,5-diarylpyroles, 2,5-diarylcyclopentadienes, polyarylphenylenes, cournarins, and polyaryl-2-pyrazolines.
Most preferred photosensitizers are anthracene, 9-methyl-anthracene, 7-dimethylamino-4-trifluoromethylcoumarin, acetone, perylene, 1,5-diphenyl-3-styryl-2-pyrazoline, 1-phenyl-3-(p-methoxy-styryl)-5-(p-methoxyphenyl-1,2-pyrazoline), isopropylthioxanthone.
The amount of photosensitizer in the photopolymerizable composition is classical and ranges generally from 0.005 to 1% by weight of total weight of photopolymerizable monomers.
The photopolymerizable monomer compositions of the invention may comprise solely as polymerizable monomers one or more of the above defined diepisulfides. However, as will be indicated later, the photopolymerizable monomer compositions may also include other polymerizable monomers different from the diepisulfides.
These monomers are compounds having two or more functional groups capable of reacting with an episulfide group or compounds having one or more of these functional groups and one or more of other homopolymerizable groups.
These additional monomers include epoxy compounds, polythiols, unsaturated ethylenic compounds such as vinyl ether or (methy)acrylate compounds, polyvalent carboxylic acids and anhydrides, thiocarboxylic acids, thio alcohols, thiophenols, polyphenols, amines and amides.
Preferred additional monomers are epoxy compounds polythiol, acrylate and (meth)acrylate compounds.
Among the preferred epoxy compounds there may be cited the compounds of formula: 
where X is O or S, Rb is H, alkyl or aryl and preferably alkyl or aryl, and R1, R2, R3, R4, m and n are defined as above.
Typical examples of the unsaturated ethylenic compounds include ethylene glycol dimethacrylate, propylene glycol dimethacrylate, trimethylene glycol dimethacrylate, tetramethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, trimethylolpropane triacrylate, pentaerythritolpropane trimethacrylate, ethoxylated bisphenol A diacrylate, 1,6-hexanediol dimethacrylate, urethane diacrylate, epoxy diacrylate, dially phthalate, and divinyl benzene.
Examples of polythiol compounds include 1,2,3-trimethylolpropane tri(thioglycolate), pentaerythritol tetra(thio-glycolate), pentaerythritol tetra(3-mercaptopropionate), 1,2,3-trimethylolpropane tri(3-mercaptopropionate), thioglycerol, dithioglycerol, trithioglycerol, dipentaerythritol hexa (2-mercapto acetate), and 3,4,5,6-tetrachloro-1,2-dimercapto benzene.
Among the (meth)acrylate compounds, there may be cited alkyl (C1-C6) (meth)acrylate such as methyl(meth)acrylate, ethyl (meth)acrylate, propyl(meth)acrylate and butyl(meth)acrylate
The additional monomers usually represents 0 to 80% by weight, preferably 0 to 50% by weight, of the total weight of polymerizable monomers present in the composition.
The photopolymerizable composition may also include other optional ingredients classically used in the formulation of polymerizable compositions such as mold release agents, antioxidants, dyes and UV light absorbers. These and other ingredients may be employed in their customary amounts for their customary purposes.
Diluents or solvents may also be added to adjust the viscosity of the polymerizable composition.
The preferred solvents are dimethylsulfoxide (DMSO), dimethylfornamide (DMF), acetonitrile and tributylphosphate (TBP).
The process of the invention includes a step of partially polymerizing the photopolymerizable composition by irradiating the composition with an UV radiation.
The ultraviolet radiation used in the photopolymenization step can be selected from any suitable source, including low, medium or high pressure mercury lamps, laser, xenon, luminescence such as fluorescence or phosphorescence, and the like. The irradiation intensity may vary from 5 mW/cm2 to 350 mW/cm2 depending upon the photopolymerizable composition and the desired degree of photopolymenization.
As previously indicated, the temperature of the exothermic photopolymerization reaction is preferably monitored by turning on and off the irradiation in order to keep the reaction temperature within a predetermined range. This temperature range usually varies from 30xc2x0 C. to 100xc2x0 C. typically 60-80xc2x0 C.
Monitoring of photopolymerization of a composition is disclosed in U.S. Pat. No. 5,422,046.
Preferably, the process of the invention includes a step of preheating the polymerizable composition. Preheating can be carried out by using a water bath, vat, IR-radiation or by an air-oven.
The most preferred process of the invention includes preheating the composition generally at a temperature of 50xc2x0 C. to 100xc2x0 C., typically 60xc2x0 C. to 80xc2x0 C., before the partial photopolymerization step and completion of polymerization by heating to a temperature of 30xc2x0 C. to 100xc2x0 C., preferably of 60xc2x0 C. to 80xc2x0 C.
The partial photopolymerization step may comprise a single pre-polymerization step resulting in the obtention of a gel or a first pre-polymerization step for forming a gel and a second polymerization step for increasing the polymerization rate beyond the mere obtention of a gel.
Subsequent to partial polymerization by radiation, the final step of the process involves a thermal heating to complete the polymerization. This is done either in an oven or using an IR source.
This heating step generally lasts for 1 to 5 hours in order to achieve complete polymerization, and typically is of 3 hours.
The process of the invention is advantageous for making plastic lenses having refractive indices as high as 1.7 or greater.
It can be implemented using classical two part glass molds.
When using the preferred process including preheating step and final heating step, the resulting lenses do not exhibit striations.
The following examples illustrate the present invention.